At present, two of the key stresses that determine atmospheric CO2 uptake by ecosystems in western North America are climate warming and widespread tree mortality due to mountain pine beetle outbreaks. Our past research of regional carbon budgets has shown that mountain forest ecosystems, sustained by the melt water from winter snowpacks, are the principal sites of carbon sequestration in the Western US. Our research in the subalpine forest at the Niwot Ridge AmeriFlux site in Colorado has shown that warmer winters over the past five decades, with associated decreases in winter snowpack, have likely caused reduced forest CO2 uptake. In 2009, the Niwot Ridge forest showed evidence of mountain pine beetle infection, with the most stressed trees the first to be attacked. We have just completed eleven years of continuous measurement of forest-atmosphere CO2 exchange in the absence of beetle infection. Now, we have the opportunity to study CO2 exchange patterns as the infection spreads, and ultimately, as the forest recovers.
We propose to use this opportunity to better elucidate the changes that will occur in soil carbon pools, as a result of tree mortality due to beetle infection, and the ease by which those pools release CO2 to the atmosphere. We will utilize forest plots at two sites – the Niwot Ridge AmeriFlux site and the Fraser Experimental Forest site, both in Colorado. At the Niwot Ridge site we will use a series of plots on which trees have been killed by simulated beetle attack over the past 8 years. At the Fraser Forest site we will use forest plots that have experienced widespread tree mortality due to natural beetle outbreaks. We propose to use advanced analysis techniques utilizing stable isotope (13C and 12C) dynamics in atmospheric CO2, radioactive isotope dating (using 14C in soil organic matter) and pyrolysis-gas chromatography-mass spectrometry to analyze patterns of soil CO2 release and changes in the soil carbon pools as a result of tree death due to simulated or real beetle attacks. The results from these observations will then be used to modify a computer simulation model in which we can explore the potential interactions between future beetle outbreaks and climate change in the Rocky Mountain region, particularly with regard to the effect of these stresses on atmospheric CO2 uptake by forests. This computer simulation analysis will allow us to better assess the potential for western U.S. forests to remove CO2 that is emitted by the combustion of fossil fuels, from the atmosphere given potential future changes in the climate and frequency of beetle outbreaks.
In particular, we will test four hypotheses:
H1. Heterotrophic soil respiration (Rh) will increase following beetle-induced tree death and, within a single decade, compensate for the loss of autotrophic soil respiration (Ra).
H2. Following beetle outbreaks, the mean radiocarbon age of respired CO2 increases, and the spectrum of soil compounds used as respiratory substrates reflect that greater age.
H3. The 13C/12C ratio of ecosystem (delta 13CR) and soil respiration (delta 13CS) will reflect an increase in the ratio of Rh to Ra following beetle outbreaks. The change in delta 13CR and delta 13CS and their relation to Ra and Rh will be responsive to seasonal and interannual climate variation.
H4. Within the first year following a beetle outbreak, summertime soil respiration rate (Rs) will be lower, but more sensitive to climate warming, due to greater temperature sensitivity of recalcitrant respiratory substrates; this trend will change over time (several years) favoring progressively higher Rs, but with continued higher sensitivity to climate warming.
The products of this research will support the DOE BER Long-Term Measures in several ways: (1) development of long-term records of the stable-isotope composition of CO2 and CO2 concentration in a forest ecosystem; these records can be used in the future to support modeling of forest-atmosphere CO2 exchange; (2) development of new knowledge about how forest loss of CO2 to the atmosphere is affected by climate change and beetle outbreaks; this knowledge will permit us to frame better hypotheses about how effectively forests will sequester CO2 from the atmosphere in the future; (3) development of the SIPNET ecosystem process model. The SIPNET model is ideally suited for the analysis of CO2 flux data from tower flux networks, and is easily coupled to regional land surface models. Coupling of the SIPNET model to regional land surface models in the future will permit exploration of interactions between forest carbon sequestration and beetle disturbances in the face of climate change in western North America.